Mandarin biochar-CO-TETA was utilized for Acid Red 73 dye adsorption from water, and its isotherm and kinetic studies were investigated

Environmental pollution is a major issue today due to the release of dyestuff waste into the environment through industrial wastewater. There is a need for affordable and effective adsorbents to remove harmful dyes from industrial waste. In this study, Mandarin biochar-CO-TETA (MBCOT) adsorbent was prepared and used to remove Acid Red 73 (AR73) dye from aqueous solutions. The efficiency of dye removal was influenced by various factors such as solution pH, contact time, initial AR73 dye concentration, and MBCOT dosage. All experiments were conducted at 25 ± 2 °C, and the optimal pH was determined to be 1.5. The optimal conditions for dye removal were found to be an AR73 dye concentration of 100 mg/L, an MBCOT dosage of 1.5 g/L, and a contact time of 150 min, resulting in a 98.08% removal rate. Various models such as pseudo-first-order (PFO), pseudo-second-order (PSO), film diffusion (FD), and intraparticle diffusion (IPD) were used to determine the adsorption kinetics of AR73 dye onto MBCOT. The results showed that the PSO model best explains the AR73 dye adsorption. Furthermore, Langmuir and Freundlich's isotherm models were studied to explain the adsorption mechanism using experimental data. The adsorption capacities at equilibrium (qe) in eliminating AR73 dye varied from 92.05 to 32.15, 128.9 to 65.39, 129.25 to 91.69, 123.73 to 111.77, and 130.54 to 125.01 mg/g. The maximum adsorption capacity (Qm) was found to be 140.85 mg/g. In conclusion, this study demonstrates that biochar produced from mandarin peels has the potential to be an effective and promising adsorbent for removing AR73 dye from water.

H 2 O, KMnO 4 , or (NH 4 ) 2 S 2 O 8 ) to biochar can enhance the number of functional groups present in it [38][39][40] .Biochars that incorporate nanometals have been shown in several studies to have excellent thermal stability, enhanced oxidation resistance, increased surface area, and expanded adsorption sites.The most often used reducing agents are FeSO 4 , H 2 , NH 3 H2O, Na 2 SO 3 , and aniline 35,41 .Nowadays, active substances for adsorption studies are obtained from agricultural wastes.The reasons for obtaining carbon from agricultural waste are the high price of commercial activated carbons, their organic nature, easy availability, non-threatening nature to health, and cost-effectiveness.Since agricultural wastes used as activated carbon give excellent results in purifying water resources, these studies have gained significant momentum.These include the peels or seeds of many agricultural materials such as oranges 42,43 , mandarin 44,45 , lemons 46 , grapefruits 47 , pomegranates 48 , bananas 49 , apricots 50 , eggs 51 , walnuts 52 , peanuts 53 , hazelnuts 54 , rice 55 , olives 56 and Palm shell 57 .
Mandarin is a popular citrus fruit that belongs to the Citrus reticulata species in the Rutaceae family.It grows in temperate climates and is known for its seedless structure, sweetness, and ease of hand peeling.Mandarin is a rich source of different chemical components such as antioxidants, dietary fibre, essential oils, flavonoids, carotenoids, and vitamin C. Due to these components, it has various medicinal properties such as reducing oxidative stress, promoting digestive health and immunity, lowering blood pressure, and even anti-cancer properties 58 .
The Food and Agriculture Organisation of the United Nations issued figures in 2021 that predicted the annual citrus output to be 80 million tonnes.China is the top producer, followed by Turkey in second place, Brazil in third place, Egypt, Japan, Spain, South Korea, and Italy 59 .After being utilized, peels from mandarin fruits are discarded into the environment, accounting for 8-14% of the fruit used to make fruit juice.The fertilizer, solid fuel, animal feed, and cosmetic sectors are the main companies that use them 60,61 .Due to mandarin being used so widely, a large volume of fruit peel ends up as biomass waste.Because mandarin peels include elements of organic carbon (hemicellulose, cellulose, lignin, and pectin) in their structure, they can be pyrolyzed to produce ecologically benign biochars 62,63 .High adsorption capacity materials may be produced using this method.
The study aimed to investigate the effectiveness of MBCOT produced from mandarin peel waste as an adsorbent for removing AR73 dye from water.The process involved dehydrating the mandarin peel waste with 85% H 2 SO 4 , oxidizing it with H 2 O 2 , and aminating it with triethylenetetramine.The resulting product, known as Mandarin biochar-CO-TETA (MBCOT), was tested under different conditions such as starting dye concentration, pH level, duration of interaction between MBCOT and AR73 dye, and MBCOT dose.To determine the maximum adsorption capacity (Q m ) and the adsorption process for the AR73 dye adsorption by MBCOT, the kinetic and isotherm models were used.This study offers a promising solution for utilizing agricultural waste, such as mandarin peel, to produce an effective and inexpensive adsorbent for removing harmful dyes from water.To the authors' knowledge, this is the first time to use MBCOT for AR73 dye removal.

Materials
For MBCOT production, Mandarin (Citrus reticulata) peels were obtained from a local market (Alexandria, Egypt).Sigma Aldrich provided the Sulfuric acid (H 2 SO 4 , M.W. = 98.07 g, 99%), Triethylenetetramine (TETA) and AR73 dye (Fig. 1).One liter of distilled water was used to dissolve one gram of dye to prepare the AR73 dye standard stock solution.

Fabrication of MBCOT
Mandarin orange peels (Citrus reticulata) were thoroughly cleaned with water multiple times to remove any dust and then dried at 115 °C for 72 h.The dried peels were pulverized and crushed for this recipe.The experiment involved boiling 250 g of crushed mandarin peels in 2L of 85% H 2 SO 4 for 5 h at ~ 240 °C in a reflux system.The peels were then diluted with deionized water, filtered, and repeatedly rinsed with water until the resulting filtrate reached a neutral consistency.After that, they were washed with ethanol, dried at 120 °C for the entire night, and weighed to produce 105 g of biomass-based biosorbent biochar made of mandarin.This biosorbent was transferred to 400 mL of 50% H 2 O 2 and then heated at 70 °C for 60 min, producing 70 g of mandarin biochar-COH biosorbent after filtration, cleaning, and drying at 115 °C overnight.The mandarin biochar-COH biosorbent (50 g) was refluxed (280 °C) in 120 mL of TETA solution for two hours, resulting in 62 g of the product after filtration, and washing twice with ethanol and distilled water.The synthesis procedures of MBCOT are illustrated in Fig. S1.The product was labelled MBCOT and the characterization was found to be similart to that previously reported by us 61,64 , and all the characterizations data were reported in the supplementary data as Figs.S2-S15.

Batch adsorption procedure
A stock solution of AR73 dye was created by dissolving 1.0 g of the dye in one litre of distilled water, resulting in a concentration of 1000 mg/L.The standard solution used for calibration and absorption experiments was produced by diluting the stock solution to the required concentrations.The absorption potential, isotherm and kinetic properties of MBCOT were determined through bulk adsorption studies.A series of 300 mL Erlenmeyer flasks were filled with 100 mL of AR73 dye solution at concentrations of 50, 100, 150, 200, and 250 mg/L, along with various doses of MBCOT at 0.50, 0.75, 1.0, 1.25, and 1.5 g/L at 25 °C.The flasks were shaken at 200 rpm for a predetermined amount of time, with the pH of the solution adjusted using 0.1 M HCl or 0.1 M NaOH, as required.Using spectrophotometry at λ max = 518 nm, the concentration of the AR73 dye was determined after a 0.5 mL sample of the solution was taken out at different times until equilibrium was established.All experiments were repeated three times and the mean values were reported and the standard deviation was ≤ 2.5.The average data for kinetic and isotherm investigations was used to conduct the adsorption experiments three times.Using Eq. (1), the adsorption capacity at equilibrium (q e ) was obtained.
where the adsorbent's ability to adsorb AR73 dye from a solution at a specific time is known as its capacity for adsorption (q t , mg/g).C 0 (mg/L) is the initial AR73 dye concentration; C t (mg/L) is the residual AR73 dye concentration after a certain period.Equation (2) may be used to calculate the percentage of AR73 dye removed from water.
To assess how pH affects the adsorption of AR73 dye by MBCOT, 0.15 g of MBCOT was mixed with 100 mL of 100 mg/L initial AR73 dye concentration.Different initial pH values ranging from 1.5 to 12.2 were studied, and to modify the pH values, 0.1 M HCl or NaOH solutions were used.Before the mixture was analyzed for AR73 dye concentration, it was stirred at 200 rpm for 2.5 h at 25 °C.Isotherm studies were conducted by shaking 100 mL of solutions with varying starting concentrations of AR73 dye (50, 100, 150, 200, and 250 mg/L) at 200 rpm for three hours at 25 °C with different quantities of MBCOT (0.5 to 1.5 g/L).To examine the effects of MBCOT dose and contact duration on AR73 dye adsorption, 100 mL of AR73 dye was shaken with different doses of MBCOT (0.50, 0.75, 1.0, 1.25, and 1.5 g/L) at 25 °C.
Author statement for the use of plants.In this work, experimental research and field studies were conducted on the waste of mandarin orange peels (Citrus reticulata).The collection of plant peel waste was carried out in compliance with relevant institutional, national, and international guidelines and legislation.

Effect of pH
The adsorption of biochar is significantly influenced by the pH of a solution.This is because it has an impact on the surface's carboxyl, hydroxyl, and amino groups.Using 1.5 g/L MBCOT adsorbent and 100 mg/L starting dye concentration at room temperature, the adsorption of AR73 dye was tested at various pH values ranging from 1.5 to 12.2 for two hours to study this impact.According to the results, which are displayed in Fig. 2a, the highest removal of AR73 dye (98.8%) happened at pH 1.5 while employing MBCOT.The adsorption rate dropped from 98.8% to 12.5% when the pH value progressively rose from 1.5 to 12.2.Between pH 1.5 and pH 7.12, the fraction www.nature.com/scientificreports/ of adsorption removal decreased dramatically; between pH 7.12 and pH 12.2, it decreased marginally.The best dye adsorption at pH 1.5 (protonation of sorbent sites-H + ) was caused by the electrostatic interaction between the negatively charged anionic dye molecules and the positively charged MBCOT surface sites.Nevertheless, as the system's pH rose, the proportion of AR73 dye molecules adsorbed to the adsorbent dropped because the MBCOT ( -OH) sites were deprotonated.The negatively charged anionic AR73 dye molecules and the negatively charged areas on the MBCOT surface were repelled by electrostatic forces as a result.Because of the adsorbent (MBCOT) surface properties and the ionisation or dissociation of the sorbate (AR73 dye) molecule, the pH of the system also affects the sorptive adsorption of sorbate molecules.It is possible to ascertain the pH at which the adsorbent surface demonstrates net electrical neutrality using the point of zero charge (pH PZC ).The pH PZC was discovered to be 10.7, as seen in Fig. 2b.The sites on the sorbent surface were positively charged if the pH was less than the pHPZC; otherwise, they were negatively charged.These results are in line with our previous study 61 .

Effect of contact time
Equilibrium time is one of the most important properties of inexpensive adsorption systems.The brief equilibrium duration demonstrates that adsorption was effective right away.When the pH was 1.5, the initial AR73 dye concentrations varied from 50 to 250 mg/L, and the MBCOT adsorbent concentration was 1.5 g/L, Fig. 3 shows the time-dependent elimination of the dye.After the first 15 min, the adsorption process picks up speed and increases steadily.Figure 3 demonstrates that the first 15-30 min account for roughly 79.44-92.19% of the adsorption of AR73 dye.
As the contact duration increased, the quantity of AR73 dye that was removed grew steadily.After 2.5 h, the removal was 96.44, 98.08, 91.69, 83.83, and 75%, respectively, depending on the starting concentration (50-250 mg/L).Most of these ions will be able to cling to the adsorbent because of the low dye concentration of www.nature.com/scientificreports/ the empty active sites in the removal of AR73 dye with a low beginning concentration of the MBCOT adsorbent.However, the removal percentage of AR73 dye will stay low when a high initial MBCOT adsorbent concentration is employed because the empty active sites are limited in their ability to adsorb further dye after being loaded with a certain amount of AR73 dye.The adsorption process occurs quickly and proves to be cost-effective in industrial applications.

Impact of initial AR73 dye concentration
To investigate the effect of the initial concentration of AR73 dye on the adsorption capacity at equilibrium (q e ), it is essential to determine the initial concentration of the adsorbed material.The effects of MBCOT dose on equilibrium adsorption capacity (q e ), 0.5-1.5 g/L MBCOT concentration and 50-250 mg/L initial AR73 dye concentration were studied at 25 °C and solution pH 1.5.The q e quantity of AR73 dye adsorbed at the same starting concentration of AR73 dye was found to rise at equilibrium as MBCOT doses dropped, as shown in Fig. 4. Using MBCOT adsorbents at several dosages (0.5-1.5 g/L), the adsorption capacities at equilibrium (q e ) in eliminating AR73 dye were established The adsorption capacity (q e ) of AR73 dye on MBCOT is influenced by the starting concentration of the dye in the equilibrium state, as shown in Fig. 4. It was discovered that q e dropped as the adsorbent dose rose.Thus, the initial concentration of AR73 dye dictated how well it adsorbs from its aqueous solution.This conclusion is supported by similar research in the literature 33,44 .The boundary layer effect is the first thing that AR73 dye molecules experience when they adsorb on MBCOT adsorbent.The molecules then diffuse from the boundary layer film to the surface of the adsorbent, where their porous nature finally causes them to adhere.

Impact of MBCOT dosage on AR73 dye adsorption
The adsorbent dosage is a significant aspect in determining the total cost, as well as the recycling and reuse costs, of the adsorption process.As shown in Fig. 5, the effect of MBCOT dosage on AR73 dye removal was investigated by adjusting the initial AR73 dye concentration (50-250 mg/L), MBCOT dosages (0.5-1.5 g/L), solution pH to 1.5, temperature (25 ± 2 °C), and contact time (150 min).While AR73 dye removal reached over 74% removal within 15 min, it gradually increased with advancing contact time.The maximum AR73 dye removal percentage and the lowest adsorption quantity at equilibrium (q e ) were discovered using a 1.5 g/L MBCOT dosage.

Adsorption ısotherms
In this step of the study, the MBCOT adsorbent was used to adsorb AR73 solutions with different initial concentrations ranging from 50 to 250 mg/L.Equilibrium studies were conducted using 0.15 g of MBCOT dosage, a solution initial pH of 1.5, and a temperature of 25 ± 2 °C.The Langmuir and Freundlich isotherms were employed to determine the equilibrium quantities for AR73 adsorption, as shown in Fig. 6.The Langmuir isotherm model is the most well-known model for monolayer adsorption.Its equation is given in Eq. 3 65 : q e (mg/g) adsorption capacity at equilibrium; C e (mg/L) solution concentration at equilibrium; Q m (mg/g) is the amount of substance adsorbed in the monolayer per unit adsorbent and K a (L mg -1 ) is the Langmuir constant.By drawing the graph of C e /q e against C e from the adsorption equilibrium data, Q m is calculated from the slope and K a is calculated from the shift (Fig. 6a).Heterogeneous, reversible, and multilayer adsorption is specified by the Freundlich isotherm model.Equation 4 provides the adsorption equation for the Freundlich adsorption isotherm model 66 : and n are Freundlich constants.By plotting the diagram of log q e versus log C e , the n value is calculated from the slope and the K F value is calculated from the drift (Fig. 6b).
( 3)   1 show that the AR73 adsorption equilibrium data is compatible with the Langmuir isotherm model.For the Langmuir isotherm model, the adsorption capacity (Q m ) in the monolayer was calculated as 140.85 mg/g.The Langmuir isotherm is consistent with the adsorption equilibrium results, which demonstrate the homogenous structure and similar active regions of the MBCOT surface.

Adsorption Kinetic Studies
Using data on AR73 adsorption, the kinetic behaviour of the adsorption process was tried to be elucidated.Kinetic studies were conducted using the pseudo-first-order (PFO), pseudo-second-order (PSO), film diffusion (FD), and intraparticle diffusion (IPD) models.The equations of the PFO kinetic model developed by Lagergren 67 and the PSO kinetic model developed by Ho and Mckay 68 are given in Eqs. 5 and 6. q e and q t (mg/g) are the adsorption capacity at equilibrium and at time t, respectively, and k 1 (min -1 ) and k 2 (g mg -1 min -1 ) are constants belonging to PFO and PSO kinetic models, respectively.For the PFO kinetic model, the graph of ln (q e -q t ) against t is drawn to find the constant qe from the slip value and k 1 from the slope value (Fig. 7a).In the PSO kinetic model, the graph of t/q t against t is drawn and the constant qm is calculated from the slope and k 2 is calculated from the shift (Fig. 7b).
Kinetic constants calculated using the data in Fig. 7 are as given in Table 2.As seen in Table 2, it was found more appropriate to represent the adsorption data with a PSO kinetic model.Equation 7determines the diffusion coefficient in the IPD model 69 .
where C is the intercept and K dif is the IPD rate constant (mg g -1 min 1/2 ) and the constant related to the boundary layer thickness (mg/g), respectively (Fig. 7c).The intercept C values give information regarding the thickness of the boundary layer; as the intercept increases, so does the resistance to external mass transfer 70 .The liquid FD model 71 can be used (Eq.8) when the solute molecules' passage from the liquid phase to the solid phase boundary is the most important factor in adsorption 72,73 .
where F and K FD are the fractional attainments of equilibrium (F = q t /q e ), and the FD rate constant, respectively (Fig. 7d).Due to the parameters calculated in Tables 2 and 3, the PSO model best suited the experimental data because of the final value of the linear regression coefficient (R 2 ) achieved (> 0.990).
The PSO model demonstrated that the calculated and experimentally determined q e values were remarkably close, indicating that the model could accurately describe the absorption process of AR73 dye into MBCOT.Redefining steps and diffusion did not have an impact on the IPD and FD linear plots via the liquid layer surrounding the sorbent since the linear plots did not continue through the origin of their respective plots.In addition, the IPD and FD models' R 2 values were less than the PSO model's.Thus, the absorption of AR73 dye to MBCOT sorbent was supported by the notion of chemisorption involving valency force via electron sharing or exchange between MBCOT and AR73 dye molecules.

Regeneration of MBCOT
The viability and reusability of MBCOT for AR73 dye absorption were investigated by performing desorption studies of the dye using 0.1 M NaOH and HCl as an extraction environment.As may be shown in Fig. 8, the study's percentage of AR73 dye desorption rose and then decreased with regeneration cycles.Six cycles of absorption and desorption were examined using regeneration MBCOT.Throughout the cycles, the change in adsorption and desorption was generally consistent.But after six cycles, it dropped by around 10%.It means that it can be used to remove AR73 dye from water.

Comparison of MBCOT with other adsorbents in AR73 dye removal
There are few studies using mandarin peel biochar as dye removal or adsorbent, and there is no research on AR73 dye adsorption using mandarin peel biochar.As a result, comparable research from the literature is included in Table 4, where it is presented that MBCOT adsorbent is successful in removing AR73 dye molecules from its water solution.

Adsorption mechanism of AR73 Dye by MBCOT
Figure 9 explains the likely adsorption mechanism via which MBCOT adsorbed the AR73 dye ions.According to FTIR analysis, a variety of functional groups, including C=O, -NH, hydroxyl O-H, C=N-, C-O-H and NH 2 groups, developed on the surface of the MBCOT.Because of the electrostatic interaction between the nitrogen and oxygen lone pair on the MBCOT surface and the negative charge on the AR73 dye, the adsorption mechanism of the AR73 dye ions in an acidic medium (pH 1.5) can be accomplished through physical interaction.
In an acidic environment, the surface of MBCOT picks up a positive charge, which attracts negatively charged dye molecules 26,83 .Furthermore, there is interaction between the functional groups of the positive ions on the MBCOT's surface and the negative ions in the solution.The dye molecules are more soluble at an acidic pH, which makes it easier for them to adhere to the adsorption sites and diffuse through the MBCOT's pores.Since the acidic pH of MBCOT is necessary to promote the adsorption of AR73 dye molecule onto the material surface, it is a great way to remove colour from industrial effluent.The most important mechanism is the electrostatic interaction-mediated adsorption of ionizable organic molecules to the positively charged surface of the adsorbents 84 .An aqueous solution's pH and ionic strength determine how well it draws or repels contaminants 84,85 .Moreover, the capacity of organic contaminants in industrial effluent to be adsorbed is influenced by the pH of the solution 86 .Parshetti et al. 's study 87 examined the use of food waste-derived biochar in the adsorption of cationic dyes from wastewater.They found that an alkaline pH enhanced the adsorption of cationic dyes.It was explained by the strong interaction between the positively charged cationic dyes and the negatively charged sites of the adsorbent surface 87 .It was less successful in adsorbing cationic dyes, though, because there was an excess of H + at pH 1.5, which competed with the positive charges of the cationic dye 87 .The capacity of organic and inorganic pollutants from industrial effluent to be adsorbed on biochar is hence influenced by the pH of the solution, which also affects the charged sites [88][89][90][91][92][93][94][95][96][97] .Tsai and Chen 98 and Xu et al. 99 have noted that pH has an impact on biochar's capacity to absorb materials.

Conclusion
In this study, we developed an adsorbent material using biochar obtained from mandarin peels.We tested the biochar (MBCOT) from mandarin peels for its ability to adsorb AR73.We examined various operating parameters such as pH, initial concentration of AR73 dye, MBCOT adsorbent dosage, and contact time in the adsorption process.The main findings are listed below: • The maximum removal of 98.08% was achieved at 25 ± 2 °C, pH of 1.5, 100 mg/L AR73 dye concentration, 1.5 g/L MBCOT adsorbent dose, and 150 min contact time.
• The PSO kinetic model matched the adsorption kinetics, and the most suitable Langmuir isotherm was the adsorption isotherm.

Figure 8 .
Figure 8.The regeneration cycles effect of MBCOT using 0.1 M NaOH and 0.1N HCl showing the percentage of AR73 dye that was desorption as well as the AR73 dye adsorption cycles using 1.0 g/L MBCOT and 100 mg/L of AR73 dye concentrations.

Figure 9 .
Figure 9.The adsorption mechanism of AR-73 dye by a possible structure of MBCOT adsorbent in acidic medium.

Table 4 .
Comparing MCO 2 T with different used materials for the removal of AR73 and different dyes.